The neurons of the mammalian hypothalamic magnocellular neurosecretory system (MNS) exhibit unusually vigorous regenerative capabilities. It has been known for many years that they can regenerate axons severed by pituitary stalk section, and this laboratory has recently demonstrated that the axon terminals of uninjured MNS neurons will also undergo robust compensatory sprouting in the adult rat. Following destruction of one half of the hypothalamo-neurohypophysial tract via a unilateral hypothalamic lesion, axons of intact contralateral MNS neurons grow sprouts which return the axon population of the neural lobe (NL) to near normal levels within 90 days. The long-term objective of this project is to determine what special characteristics of the MNS provide the basis for this plasticity. This proposal builds upon our recent findings that (a) both vasopressin (VP) and oxytocin (OT) neurons appear to be hyperactivated during the sprouting response and increase expression of alpha1 and betaII tubulin isoforms; and (b) that transient glial activation occurs in the NL immediately prior to the initiation of sprouting, while continuous angiogenesis accompanies sprouting. The Specific Aims of this proposal are: (1) to test the hypothesis that increased neuronal activity facilitates collateral sprouting, whereas decreased activity inhibits it. This will be accomplished by correlating neuronal activity estimated by measures of VP and OT gene expression, neurosecretion, and cellular metabolism with the extent of axonal sprouting in juvenile and adult rats under different osmotic conditions known to alter neurosecretory activity; (2) to test the hypothesis that collateral sprouting by MNS neurons requires upregulation of alpha1, betaII, and betaIII tubulin mRNAs. This will be accomplished using in situ hybridization (ISHH) to correlate relative changes in mRNA pools with the extent of collateral sprouting; and (3) to test the hypothesis that basic Fibroblast Growth Factor (bFGF), produced by and acting upon different cell types in the MNS, serves to stimulate and coordinate axonal sprouting and associated angiogenesis in the NL. This will be accomplished using ISHH and ICC techniques to detect expression of bFGF and its high affinity receptor FGF-R by cells of the MNS during axonal sprouting and correlated angiogenesis. Fulfillment of these aims will provide new insights into basic cellular mechanisms which determine the extent of collateral sprouting by central peptidergic neurons. Since virtually all neural disorders involve some degree of tissue degeneration, increased knowledge of the cell biology underlying compensatory responses to injury of the CNS is crucial for the ultimate design of effective therapies.